The FBX SDK specifies that normals should use the IndexToDirect FBX
reference mode. This mode allows data written to FBX to be deduplicated
by writing a separate array of indices into the deduplicated array of
data that can be used to reconstruct the full array of data.
When Unity imports per-vertex normals with the IndexToDirect reference
mode, it erroneously considers the imported mesh's normals invalid if
any of its normals were deduplicated. This issue is not present in other
external software that I have tested.
This patch modifies the export of per-vertex normals to not perform any
deduplication, avoiding the issue in Unity's FBX importer while
maintaining use of the IndexToDirect reference mode.
The add-on's patch version has been increased.
Pull Request: https://projects.blender.org/blender/blender/pulls/123186
Thomas Barlow
b899fbbe1c